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close this book Boiling Point No. 13 - August 1987
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"The kitchen kills more than the sword.. (German proverb)

Kirk R. Smith, Biofuels and Development Project, East-West Centre, Honolulu, Hawaii 96848

While there are many anecdotal accounts linking smoky stoves to lung and eye problems, few scientific studies have directly addressed this question. This is partly because of the difficulties of doing such research in a systematic manner and partly because of a lack of concern in the scientific and medical establishments that conduct such work. This lack of concern was not without some justification in the past. As with other traditional forms of pollution such as water contaminated by human waste, there may have seemed to be little argument about the need to eliminate the heavy exposures characterizing less-developed regions. Since in the past there has been a natural evolution away from open biofuel-fired stoves during economic development, it may thus have seemed unnecessary to spend much effort to characterize the relationship between smoke exposures and health effects that were, in any case, on the way out.

The changes in relative fuel costs and availabilities characterizing the 1970s, however, led to different perceptions of the evolution of domestic energy use. It is now thought that biofuels may well have a relatively long future in a large percentage of the world's households. There are a number of implications of this view. The most obvious is that in most areas the biomass fuel cycle will have to change. Managed production must replace the unmanaged "hunt-and-gather" techniques relied upon for harvesting most of today's household fuels. In addition, to serve development as well as survival needs, there will be need for a greater degree of upgrading to higher quality fuels such as charcoal, gases, and alcohols. Finally, of course, the fuel cycle must end with devices that achieve higher efficiencies if biofuels are both to be harvested on a sustainable basis and to continue to meet household fuel demands.

There are also implications for health and safety. No more can it be expected that existing problems will go away by themselves. They must be directly addressed at each step of the fuel cycle. Because there may well be difficult trade-offs among the desires for economy, efficiency, cleanliness, and other characteristics, increased quantification of the impacts on health will be required to make rational choices.

Such factors as economy, efficiency, and, to some extent, safety are fairly easily perceived by the users themselves. It can thus be argued that, given the opportunity, they are best qualified to choose among alternatives in a way that best serves their own interests. Environmental contaminants, however, present a more difficult problem. Their impacts are often delayed and otherwise difficult to link directly to exposures. Indeed, some of the most damaging pollutants cannot be perceived at all by human senses. Neither are the health effects easily distinguishable from those with other causes. Thus, to pin down effects, it is necessary to rely on instrumentation, statistical judgements, and expert opinion. This is sometimes even true when the effects are great, as they are, for example, with tobacco smoking.

How widespread is biofuel usage?

The readers of 'Boiling Point' know already that biofuel is the most important household fuel in the world. As has been true since the discovery of fire, most people rely most of the time on such fuels for household energy needs. While it is difficult to be precise ant there are many local and seasonal variations, it seems that the vast majority of such use occurs in open stoves that do not vent the smoke away from the user.

The purpose of this paper, therefore, is to answer the question:

What health impact is created by biofuel cookstove smoke exposures?

To answer directly, unfortunately, is not possible at this time because so little direct work has been done. It is possible, however,-to break down the question into a series of subquestions that can be partially answered and, taken together, give some indication of the extent of the problem. Figure I shows the framework within which the components can be linked


How much smoke is produced by biofuel combustion?

While in some circumstances biofuels can be burned with little smoke production, it is quite difficult to do so in simple small-scale stoves. Instead, a wide range of pollutants are normally emitted in several major categories: nitrogen dioxide, carbon monoxide, particulates, and hydrocarbons. The rate of production is generally substantially higher than from the combustion of gasious or liquid fuels and is only rivalled by burning of other high-volatile solid fuels such as coal.

How dangerous is biofuel smoke compared to that from other fuels?

Biofuel smoke like tobacco smoke, contains hundreds and probably thousands of individual carbon containing chemicals, many of which have been shown to be damaging to health in either human or animal studies. These include potentially cancer causing polyaromatic hydrocarbons, such as benzo(a)pyrene, aldehydes, such as formaldehyde, and aromatics such as benzene. In addition, the size of the particles is such that they can penetrate into the deep lungs where it is thought the most damage can occur. Laboratory studies show that, gram for gram, woodsmoke seems to have about the same potential for producing tumours as smoke from the burning of tobacco or vehicle fuels. This contrasts, for example, to the smokes from certain coals that seem to have much greater activity.

How much smoke are people exposed to?

Only a small number of studies have been done in which actual exposures have been measured. Indeed, it has only been in recent years that the techniques and equipment for such studies have been developed, largely in response to the growing concern about indoor air quality in developed countries. Only a limited number of geographic and social conditions, fuels, stoves, house types, and time periods have been studied. No standard methods have yet been developed. For example, most measurements have been done by stationary monitors rather than by personal monitors, which are actually worn by the householders. Since there are wide variations of smoke concentration in different parts of the house at different times, it is difficult to determine actual human exposures from such measurements.

(See Figure I )

In general, where open combustion occurs indoors, such measurements have found high levels of particulates and some of the limited number of organic compounds that have been measured. Typical conditions greatly exceed concentrations in any but the dirtiest urban outdoor environments. Maximum levels exceed anything measured in cities. Carbon monoxide and nitrogen dioxide levels are often above the standards set to protect public health, but, except very near the fire, not to the extent demonstrated by particulates and organic compounds. Compounds that have caused cancer in animals and are known to be in mixtures that cause cancer in humans (e.g. tobacco smoke and coal tar) have also been found at extremely high levels compared to urban situations.

It has been difficult to strongly associate exposures with household parameters, such as volume and area of windows. Roof type does seem to have an effect as well as the proximity to nearby houses. Statistical tests tend to show greater exposure variation within homes than between homes with the same stove and fuel characteristics. In general, however, people in households at higher elevation will have higher exposures because of increased use of the stove for space heating and, possibly, tighter ventilation conditions.


What health effects would be expected and what has been found?

Based on studies of the same compounds but in different mixtures, populations, and exposure patterns (i.e. cities, occcupational settings, and tobacco smoking), there are four major types of health effects that might be expected from such exposures:

Figure 1. Framework for organizing the study of the health impacts of biofuel smoke from household combustion. Note the inclusion of indirect benefits, mainly mosquito repellence and thatch preservation. The notation, 'R', indicates those points where technical fixes are most readily available.

Acute respiratory infections (ARI): The most conclusively demonstrated health effect of passive tobacco smoking is the increase of ARI in children. In village houses, however, typical exposures to most pollutants greatly exceed the levels resulting from passive smoking exposures in developed countries. Preliminary studies in rural Nepal have shown a relationship, between hours per day spent near the stove and the incidence of severity of ARI among very young children. A study in Papua New Guinea failed to find a relationship for school-age children. It is important to understand this connection since ARI is now one of the two principle causes of illness and death among the world's young children even exceeding diarrhoea in many estimates. There are a number of risk factors of which smoke exposure may be important in some regions.

Chronic obstructive lung diseases (COLD): Chronic bronchitis and other forms of COLD have been associated with long term exposures to air pollution of various kings, including active tobacco smoking. Preliminary studies in Nepal and India have shown an association of COLD with cooking for women, while other studies in Papua New Guinea have led to conflicting results. Such studies are difficult, however, because of the need to determine the history of exposure over many years.

Low birth weights: Evidence from both active and passive smoking studies indicates that the pollutant exposures to pregnant women in village households may be sufficient to be a factor along with nutrition and other influences in low birthwight. Low birthweight, of course, is highly correlated with infant mortality and lifelong disability. No studies of the impact of household smoke, however, seem to have been done.

Cancer although containing a number of suspect carcinogens, there seems to be little concrete evidence of excess cancer from biofuel smoke. In the past, some studies in Kenya and among various populations of southern Chinese have pointed to nasophraryngeal cancer but more recent analysis has downplayed the role of stove smoke. Age-adjusted lung cancer rates are generally thought to be low in those areas of the world with high biofuel smoke exposures, although no systematic studies of the connection seem to have been done. Recent evidence from China does point to a possible impact but not nearly to the extent as smoke from the local coal burned in village stoves. The impact of biofuel smoke exposures on cancer remains elusive, although it can probably be said not to be large, in spite of large theoretical risks based on extrapolations from other known carcinogenic mixtures.

Eye problems: While anecdotal accounts of eye problems related to smoke are common, there seem to be no systematic studies of the problem. although the existing studies referred to above are suggestive, no epidemiological study has yet been done that actually measures both health outcomes as well as smoke exposures. Neither have any before-and-after (intervention) studies been done to determine the health improvement resulting from exposure reduction measures such as the introduction of improved stoves. Clearly, however, such studies are warranted given the large exposures, large population, and preliminary results of the semi quantitative studies done to date. Easiest will be studies of ARI and low birthweight in which exposures and outcomes are most closely connected in time. The wide variation in household exposures argues for utilizing a control group that is similar in all respects except smoke exposure. This is difficult in practice because so many cultural and economic parameters also correlate with smoke exposures. The best approach, therefore, is to employ studies designed to produce their own control group through intervention.


Are there positive as well as negative impacts of exposures?

Anecdotal accounts of benefits to smoke exposures are nearly as common as those describing ill effects. As indicated in Figure 1, an analysis of the overall interaction of smoke exposures and human welfare should take these factors into account. Unfortunately, it seems again, however, that few, if any, systematic studies have been done to verify and quantify these benefits.

The most important benefit ascribed to such exposures is mosquito repellence. Certainly, with the rise in malaria occurring in some parts of the world along with pesticide and quinine resistance, such a benefit needs to be carefully considered. As with the other aspects of the overall problem, however, the absence of scientific interest has meant that there are no standard methods available to test the effects of smoke on mosquito behaviour. At the EWC, work has been done to develop such a method but has not been yet applied to malaria-carrying mosquitos. Preliminary results seem to show that effective mosquito repellence can occur at smoke concentrations substantially below what is often found in village houses.

This is consistent with other anecdotal evidence based on interviews with village women who have adopted improved stoves with flues. They report the ability to continue mosquito repellence by burning small amounts of specific local biomass forms such as neem leaves. This is analogous to the use of a mosquito coil, which results in much lower concentrations of smoke but releases compounds of particular impact on mosquitos. Again, however, much more systematic research would be needed to pin down these relationships with confidence.

The second most commonly noted benefit of smoke is preservation of household thatch. Again, although certainly amenable to experimental validation, only anecdotal evidence seems to be available at present. Neither is this evidence consistent in that many surveys indicate that villagers actually perceive little such advantage to smoke, point out that fumigation with smoke can be done in ways that minimize human exposures, or describe negative impacts of smoke on household materials. More systematic investigation is clearly needed to pin down these relationships.


Even though it is not yet possible to estimate the precise amount of health effects, enough is known to warrant efforts to reduce them. An integrated approach to control includes economic, managerial, political, and social issues but, for 'Boiling Point' readers, it may be most valuable to discuss this question with regard to two types of technical fixes, changes in fuels and stoves. As with stove efficiency, there have been problems in developing standard techniques for measuring emissions of smoke from open stoves and determining exposures that may result. Individual differences in tending the fire can make large differences in both emissions and exposures. As a result, laboratory results often differ substantially from field data.

Fuel: In general, there seems to be much more variation in smoke emissions among different combustion conditions than among different types of unprocessed biofuel. Few studies have been done of the many different types of crop residues, but, under common conditions in small stoves, they seem to be smokier than wood but less so than animal dung. It does seem to be possible to make some generalizations about some physical parameters. For example, for any stove and fuel type there seems to be an optimum ratio of surface area to volume (size of fuel pieces) and optimum fuel moisture content. Fuel that is larger, smaller, wetter, or dryer than these optimums will have greater pollution emissions. (This is shown in Figure 2, for a wood heating stove.) In general, these optimums seem to lie in the range of sticks of 24 cm in diameter and air-dried moisture levels, but tests would have to be done on any one stove/fuel combination to be more specific. At the EWC, for example, stove arrangements have been found in which the highest combustion efficiency and lowest emissions occur with wood of 50% moisture.


Figure 2. The relationship of overall, combustion, and heat transfer efficiencies to fuel moisture content (dry basis). Note that the three efficiencies do not peak at the same point. Since emissions are inversely proportional to combustion efficiency, therefore, the points of greatest overall efficiency and lowest emissions do not coincide. Since few such tests have been done with biofuel cookstoves, this example is taken from a study of wood heating stoves done by J.W. Shelton at the Woodstove Research Institute, Sante Fe, New Mexico.

It is generally true to say that compared to unprocessed biofuels, household combustion of upgraded biofuels will have lower emissions. In the case of charcoal, however, this can sometimes actually create extra risk. This comes about because the combustion of low-volatile solid fuels such as charcoal and some coals produce few particulates and hydrocarbons although still producing substantial amounts of carbon monoxide (CO). CO exposure is a hazard not only on a chronic long term basis but also over the short term if exposures are high enough. Normally, however, it is impossible to succumb to CO poisoning from the smoke of wood or other forms of natural biofuel. This is because the concentrations of hydrocarbons in the smoke increases along with CO and long before CO exposures have reached dangerous levels, people will be awakened and driven from the room because of the intense irritation caused by the aldhydes and other irritants in the smoke. While many of the hydrocarbons are long term hazards, therefore, their presence can be beneficial in the short-term because they act as a warning for build-up of CO concentrations. Low-volatile solid fuels, which do not produce this hydrocarbon alarm, however, can and do cause CO poisoning; as in Korea and northern China where coal is used as heating fuel.

Thus a program to replace biofuels with a low volatile solid fuel (whether charcoal or coal) must be careful to take safety into account. The stove and ventilation conditions should be examined to be sure that CO poisoning does not occur. In addition, the public must somehow be informed of the danger because CO by itself is essentially not detectable by normal human senses.


Even less work has been done to determine how modifications in cookstove design affect emissions. Much information can be gleaned, however, from the extensive research done with wood-fired metal heating stoves that have recently become popular again in many developed countries. Indeed, many developed countries have found need to rapidly develop and promulgate rigid air pollution controls for household woodstoves because of the high emission levels characterizing most traditional designs. In the USA, for example, the Environmental Protection Agency, pushed by a lawsuit brought by the Natural Resources Defense Council, has recently announced woodstove emission standards to be enforced on new stoves next year. This has been justified because, by the mid 1980s, woodstoves had probably become the largest source of several important categories of air pollution in the country - exceeding, for example, the CO emissions of all US industry and matching the entire power industry in particulate emissions.

The concern in developed countries, of course, relates to outdoor air quality since metal heating stoves essentially all have flues or chimney. The village cookstove, on the other hand, typically does not and emits directly into the household environment. Many of the improved cookstove programs around the world have promoted stoves with flues. Sometimes these stoves are called 'smokeless' although they are not designed to emit less smoke but to direct the smoke out of the house. Indeed the most common designs probably actually increase total smoke output compared to the traditional open combustion stove.

The history of improved village stoves since mid century has been characterized by three overlapping periods. The earliest or 'Classic' period focused on reducing smoke exposures but generally did not apply scientific approaches to design, promotion, and testing. The Energy period stoves, which came about during the 1970s in response to energy environment concerns, focused on improving fuel efficiency. All too often, however, these programs also failed to apply scientific and critical methods. We are now witnessing the evolution to a new period, although programs representing both older approaches are still active.

The new period, which here will be called 'Modern', attempts to learn from the past and to incorporate the lessons learned in both earlier periods. Some of the important lessons are

- Both improved fuel utilization and reduced smoke exposures need to be considered as primary goals. Indeed, most post dissemination surveys of improved stoves introduced to areas where traditional stoves cause large exposures, have found that reduced smoke exposure is cited more often than improved efficiency as the largest benefit by users.

- Considerable engineering and market research is needed before a new model can be successfully disseminated. Field research must incorporate study designs that are capable of providing statistical statements of user perceptions and stove performance. More work needs to go into development of standard methods for measuring efficiency and exposures --under laboratory, simulated, and field conditions.

- Social niches exist for both locally made stoves utilizing mostly local materials as well as centrally made devices of metal or ceramic in which stricter quality control and economies of scale can be applied. Both welfare and market approaches to dissemination must be developed and will be appropriate in different places at different times.

- Stove programs should not expect to be able to optimize one or two aspects of traditional stoves while maintaining all their other characteristics such as portability, flexibility, zero cost, insect fumigation, and room lighting. This is not to argue that such functions are unimportant, but that they will need to be addressed by other means. Economic and technological development has nearly always been accompanied by specialization and there is no reason that evolution of the cookstove will be different.


There are trade-offs between efficiency and emissions in many stove designs. Efficiency and low exposures may seem to be, and indeed are, in general, compatible goals. After all, the source of most emissions from biofuel combustion is incomplete combustion and, thus, high combustion efficiency means low emission factors (emissions per unit fuel). Unfortunately, however, some of the principal techniques used by stove designers to increase efficiency actually increase emission factors as well. This comes about because overall stove efficiency is a combination of two separable internal efficiencies, as illustrated in Figure 2. Enclosing the combustion chamber and reducing airflow - two common approaches in improved stoves - may increase overall efficiency by increasing heat transfer efficiency (shifting the heat transfer curve upward in Figure 2). This may, however, actually decrease the combustion efficiency because of poorer turbulence and a lower air fuel ratio. The results can be, therefore, increases in both overall thermal efficiency and emission factors. Care must be taken, therefore, to improve or at least maintain combustion efficiency when seeking modifications to improve overall fuel utilization.

To predict the health impacts of changes in heat transfer, combustion, and overall efficiencies is not straightforward. This is because exposure is not a direct function of emission factors, but is also affected by the emissions rate, cooking time, room ventilation, proximity to stove, and other factors that may themselves be changed by modifications in stoves designed to improve fuel utilization (see Figure 1). In some cases, for example, an increased emission factor may be more than compensated for by a decrease in total fuel usage and cooking time. On the other hand, lower emission factors themselves do not guarantee decreases in exposures.

Thus, as with fuel utilization, laboratory and simulated tests impart only limited ability to predict actual exposures. Field tests are necessary. In addition, even field verification that fuel utilization has improved is not sufficient by itself to conclude that exposures have lowered.

What about smokeless stoves?

It might be thought that the above discussion refers only to stove improvements that do not incorporate flues. Unfortunately, this is not so. It is clear from several studies in India, for example, that the existence of a flue is not sufficient to guarantee a significant reduction in human exposures under field conditions. A number of factors seem to be involved, but, in general it is unfortunately true to say that stoves in the field are often not built, operated, or maintained in the ways intended by their designers. In addition, users may frequently substitute fuels and pots in ways that lead to smoke releases. Thus, field tests are needed to verify the extent of exposure reductions. Even user perceptions can be misleading because not all of the critical pollutants are readily sensed although surveying such perceptions is obviously important for other reasons.

Another factor that tends to limit the exposure reduction of flued stoves is the entry of smoke from outside the house. Since smoke is still produced (even, in some cases, in greater amounts) by flued stoves, the outside air can still become heavily polluted. When houses are close together, stoves are used at the same time of day, and outdoor ventilation is low (as in the dry winter season characterizing many continental areas away from the ocean) local ambient air pollution can reach high levels. In these cases, the relatively high ventilation rates of village housing can lead to a significant indoor concentration even when the flued stoves are working well. In such conditions, even homes using biogas for cooking can experience nearly as high concentrations as nearby homes using traditional fuels even though biogas combustion itself contributes very little. A study in Nepal, on the other hand, where houses were widely spaced horizontally and vertically, found significantly lower exposures among women cooking on smokeless stoves.

To be truly smokeless, stoves need to incorporate features such as secondary combustion chambers that directly decrease emissions. Unfortunately, it has turned out to be difficult to design such devices to operate reliably. This is true even for metal heating stoves in developed countries, which cost many hundreds of dollars. In what might be called the i 'woodstove dilemma', the rate of energy (power) needed by typical houses occurs just at the lower limit of wood burn rates at which high combustion efficiency and low emissions can be maintained (see Figure 3). Unfortunately, the typical power needs for household cooking creates this same problem.

To solve this dilemma in developed countries, many stove manufacturers have turned to catalytic converters. It may seem somehow inappropriate to rely on such a sophisticated device for cleaning such an ancient pollutant, but, in reality, it is ideally suited because most biofuels have so little ash and other contaminants that foul the catalysts. Apparently, therefore, the lifetime of the catalytic converters in such usage can be quite long. In addition, at least with heating stoves, the catalytic converters increase the efficiency of combustion such that, in typical developed-country conditions, they are usually quite cost effective for the users.

The cost of woodstove catalytic converters has decreased dramatically since the early 1980s, being now something less than $40. This is still too high for consideration by many developing country users, but, just as has been shown with photovoltaics, there may well be appropriate niches in developing countries for high-technology devices that are user friendly (particularly if they can be made in country). Application to cooking stoves, however, may not necessarily be easy.

A more modest approach to accomplishing the sometimes conflicting goals of low exposure and high efficiency is to optimize stove design for efficiency without a flue, but for use on a fireplace like hearth under a chimney. Such arrangements have been found to be quite effective in field tests in India, for example. In addition, the chimney arrangement can often be made of the same kind of materials used for the walls of the house itself.

Figure 3. The effect of burn rate on carbon monoxide emission factor for a wood-fired heating stove. Note the apparent increase in emissions occurring at rates less than 1-2 kg/in. These tests were performed by A.C.S. Hayden and R.W. Braaten at the Canadian Combustion Research Lab, Ottawa.


The history of the world has shown that at every occasion where alternatives have been available and affordable, people eagerly turn away from unprocessed solid fuels for cooking. Sadly, perhaps, I do not believe that the Classic and Energy periods of improved biofuel fired stoves have provided any evidence that this trend will somehow change in the future. Such fuels are simply inconvenient, dirty, bulky, hard to control, inefficient, and otherwise unsuited to cooking. It is to be hoped that the improved stoves of the Modern period will mitigate the impact of these characteristics and thus help make more comfortable and sustainable the unavoidable and substantial period that remains for reliance on such fuels by large populations. It is well worth considering, however, at what level of effort we may actually start to engage in suboptimizing and counterproductive activities by pursuing further improvements in cookstoves burning unprocessed biofuel rather than the means to accelerate the natural trends leading away from them.